Okay I would say this:
If she hasn't done anything in the past for you to reasonably suspect there's something else going on, then you have to assume there isn't. If you start getting jealous and needy and suspicious, that's a big turn off for her subconsciously and maybe consciously too. Try to contain your jealousy and remember all the things that make you guys great as a couple. Think about how unlikely it is that she would throw that away, and that she wouldn't betray your trust like that. Think about the positive stuff.
Besides, I'm sure the other things like her not talking as much sometimes means absolutely nothing. Sure, it could be because of this new friend, but it's unlikely to be for romantic reasons. It's obviously in the nature of people to prioritise those who are around them, and when she's hanging with this girl she's not going to be messaging you as much. But it doesn't mean anything.
The negative thoughts train is a dangerous one to be going down, I understand. Once you get an idea in your head it's difficult to shake. All you have to try and do is play with the percentages, and I promise you the things you're thinking of are like 99% not happening. You just need to stop yourself going down these thought paths and being influenced to call her or message her.
You just need some damage reduction. I would say that you should tell her you do trust her and put it down to the work issues and other stuff, and try your best to not get irrationally jealous. Sure, she is spending a lot of time with and talking about this girl, but if you are serious she probably isn't do anything and even if she did do something, it sounds like she'd feel too guilty and would tell you. Try to believe in and trust that, explain your problems to her (that maybe the stress and depression is making you over jealous), and you'll be fine. But you can't be calling her up and accusing her and telling her you don't trust her. That will absolutely 100% drive her away. Be more positive to her and you'll both be fine, cause it sounds like she is doing nothing wrong!